Energy storage/withdrawal system for a facility

ABSTRACT

The invention relates to a system ( 100 ) for storing/withdrawing thermal energy. 
     The main characteristic of a system according to the invention is that it comprises:
         a monolithic cementitious material ( 1 ) comprising a mass fraction of ettringite of greater than 20%, said material being surrounded by a thermal insulation material ( 12 ) and a water insulation material ( 11 ),   a source ( 2 ) of a heat transfer fluid,   a device ( 3 ) for wetting said fluid in order to carry out a withdrawal phase of the system ( 100 ),   a device ( 4 ) for heating said fluid in order to carry out a storage phase of said system ( 100 ),   an outlet ( 6 ) of the heat transfer fluid from said material ( 1 ).

The invention relates to an energy storage/withdrawal system for a facility.

The global overconsumption of energy has significant consequences at the environmental and socio-economic level. The gradual depletion of nonrenewable energies and the hike in energy prices have led to the emergence of directives that aim to increase the energy efficiency by 20% from now until 2020 in the European Union. The building sector is the most energy-consuming sector, ahead of industry and transport, the major part of this energy being consumed in calorific form in order to maintain a certain thermal comfort.

In Europe, the building sector is the highest consumer of primary energy, representing 40% of the total energy consumed, and is responsible for 36% of the CO₂ emissions, more than half of this energy being consumed in the form of heat. This sector therefore has the highest energy-saving potential.

The use of solar energy is essential nowadays to improve the energy performance of buildings and to limit their impact on the environment. However, one major drawback of solar energy is its intermittence. Specifically, in summer the solar energy exceeds the energy requirements, unlike in winter where there is a shortage of thermal energy. Seasonal thermal energy storage can prevent this phase difference and increase the use of solar energy in the building sector. Materials with a high heat capacity (sensible heat storage) and phase-change materials (latent storage) are widely used in the building sector for short-term solar energy storage.

Patent application WO2009144233 describes a system for storing sensible heat in a monolithic concrete block. One drawback of this system is that it has a low storage density. Furthermore, this system requires permanent heating and perfect insulation to prevent the heat losses that get bigger with the duration of heat storage, these two actions constituting significant constraints.

Therefore, the drawbacks linked to the systems for storing sensible heat are a low storage density at low temperature, heat loss during this storage, the bulkiness thereof and a limited storage time.

Storage systems that involve phase change materials (latent heat storage), even though they have an energy storage density greater than that of the storage systems involving materials with a high heat capacity (sensible heat storage), also have, for some of them, a certain number of drawbacks:

-   -   a low phase-change enthalpy,     -   an inflammability,     -   a fast phase change,     -   a limited storage time,     -   a volume instability

Another solution consists in using sorption heat storage systems. Sorption storage materials are highly porous materials, often in the form of minuscule porous beads (powder bed). Heating the adsorbent material leads to an endothermic desorption (rupture of the bonds between the solid and the gaseous molecules). This is the charging phase. The desorption enthalpy stored in the material is returned during the adsorption (exothermic reaction) of the desorbed molecules. This is the discharging phase. Depending on the material used, these storage process may be due to a physical and/or chemical adsorption. These storage systems have the drawback of possessing a low storage density and a short withdrawal time.

Another solution consists in using chemical heat storage systems involving the thermal energy derived from a chemical reaction in the storage material. In the charging phase, the material is heated, for example with solar energy, to the chemical decomposition thereof into two stable separate elements. The thermal energy stored, corresponding to the decomposition enthalpy, is returned during an exothermic chemical reaction of the two stable elements. Despite their high heat storage density, the main drawbacks of most of the chemical storage materials are the following:

-   -   a high storage temperature,     -   the irreversibility of the reaction,     -   a certain complexity of the storage process,     -   a significant toxicity,     -   a high cost.

Another solution consists in storing heat using ettringite, which is a mineral composed of calcium sulfate, calcium aluminate and water. This material has both the behaviour of the sorption storage materials (short-term performance) and that of the chemical reaction storage materials (long-term performance). Indeed, ettringite has the advantage of having a high thermochemical storage density at low temperature (below 70° C.) owing to the physicochemical process implemented. Heat is stored therein by endothermic heating (desorption+dehydration) of the material, which is dehydrated then becomes metaettringite. The heat can then be withdrawn by exothermic adsorption (adsorption+hydration). The use of this cementitious material as thermochemical storage material in the building sector is limited by durability, carbonation and reversibility problems.

An energy storage/withdrawal system according to the invention overcomes all the drawbacks found in the prior art.

In order to facilitate the understanding of the description, an energy storage phase comprises an active phase of charging said energy into the material and a passive phase of conserving said energy in said material. During this passive phase, the storage/withdrawal system is not the site of any thermochemical reaction and the heat transfer fluid does not circulate in said system.

One subject of the invention is a system for storing/withdrawing thermal energy.

The main characteristic of an energy storage/withdrawal system according to the invention is that it comprises:

-   -   a monolithic cementitious material comprising a mass fraction of         ettringite of greater than 20%, said material being surrounded         by a thermal insulation material and a water insulation         material,     -   a source of a heat transfer fluid,     -   a device for wetting said fluid in order to carry out an energy         withdrawal phase,     -   a device for heating said fluid in order to carry out an energy         storage phase,     -   an outlet of the heat transfer fluid from said material.

Such a system can be used, either to store heat, or to return this heat once it has been stored. Thus, in the heat storage phase, the heating device is activated to heat the heat transfer fluid in order to obtain a hot (temperature above 30° C.) and dry gas. This hot gas then passes through the monolithic cementitious material comprising ettringite, which is gradually dehydrated, giving rise to the heat storage phase. For this configuration, the hot gas interacts directly with the cementitious material by being diffused homogeneously through the whole of its volume. This narrow interaction is made possible owing to the good porosity of the cementitious material comprising ettringite. Indeed, owing to this marked porosity, it is not necessary to use a device for distributing hot gas within the cementitious material comprising ettringite, such as for example a network of pipes for circulating hot gas. During this heat charging phase, the wetting device is not needed, and the heat stored by the cementitious material is not returned while said material is not in contact with water, in liquid or vapour form. In the heat discharging phase, the wetting device is needed in order to charge the heat transfer fluid with water vapour. The resulting wetted gas passes through the cementitious material which adsorbs the water vapour, then is hydrated, generating an exothermic reaction, which then produces heat. For this configuration, the wetted gas interacts directly with the cementitious material by being diffused homogeneously through the whole of its volume. This narrow interaction is made possible owing to the good porosity of the cementitious material comprising ettringite. Indeed, owing to this marked porosity, it is not necessary to use a device for distributing hot gas within the cementitious material comprising ettringite, such as for example a network of pipes for circulating hot gas. During this heat restoring phase, the initial cold and wetted gas is converted into hot and dry gas at the outlet of the cementitious material. The passage of a gas through the material is made possible owing to the high porosity of the cementitious material comprising ettringite. It is important to ensure that carbon dioxide does not come into contact with the monolithic cementitious material, at the risk of resulting in a carbonation of the material, and therefore an irreversible degradation and then a drop in efficiency of the storage/withdrawal system. In order to prevent such prejudicial contact, the heat transfer fluid may be formed by an inert gas such as for example nitrogen. On the other hand, if the chosen heat transfer gas was caused to transport carbon dioxide, it is then necessary to make provision to trap these carbon dioxide particles upstream of the cementitious material, for example by means of a selective membrane. Provision may be made for a system according to the invention to simultaneously carry out a heat storage and withdrawal phase, in particular by creating two heat transfer fluid circulation circuits in parallel. Preferentially, an energy storage/withdrawal system according to the invention comprises at least one heat transfer fluid circulation circuit that passes through the heating device and through the wetting device, said circuit being supplied by a source of said fluid and leading to the cementitious material. It is assumed that the storage phase comprises an active phase of charging heat into the cementitious material and a passive phase of conserving this heat in said cementitious material, the heating device being activated only during this charging phase. The water insulation material constitutes a barrier intended to prevent water vapour from coming into contact with the cementitious material.

Advantageously, the heat transfer fluid is formed by an inert gas. Preferentially, this inert gas is nitrogen. Such a fluid makes it possible to avoid polluting the cementitious material with carbon dioxide.

Preferentially, the source of the heat transfer fluid is placed upstream of the wetting device, which is itself placed upstream of the heating device, said heating device being placed upstream of the cementitious material. This is one embodiment suitable for a system according to the invention.

Preferentially, the wetting device and the heating device each operate to order, and can be activated independently of one another depending on the use of said system, either in storage phase, or in withdrawal phase. The various constituent elements of a storage/withdrawal system according to the invention are necessary for these energy storage/withdrawal functions, but are not necessarily intended to operate simultaneously.

Advantageously, the mass fraction of ettringite in the cementitious material is between 20% and 90%. The ettringite must be present in massive amounts in the monolithic cementitious material in order to carry out, under optimized conditions, the heat storage and withdrawal phases.

Advantageously, the cementitious material has a porosity of between 10% and 90%.

Preferentially, the material has a permeability of between 10⁻¹⁷ m² and 10⁻¹¹ m².

Advantageously, the cementitious material has a mechanical strength of between 0.1 MPa and 70 MPa.

Preferentially, the water insulation material is formed by an impermeable wall intended to prevent water vapour from coming into contact with the cementitious material. This wall may for example be formed by PVC. This is a nonlimiting example of a water insulation material that has a good water insulation while remaining lightweight.

Advantageously, the thermal insulation material is formed by at least one material to be chosen from glass wool and polystyrene, said at least one material being placed around the water insulation material.

Advantageously, the wetting device comprises at least one humidifier suitable for charging said fluid with water. This humidifier may for example be formed by a bubbler.

Another subject of the invention is a process for storing/withdrawing thermal energy using a storage/withdrawal system in accordance with the invention.

The main characteristic of a process according to the invention is that, in the heat storage phase, it comprises the following steps:

-   -   a step of heating the heat transfer fluid in order to obtain a         hot gas, the temperature of which is above 30° C.,     -   a step of directly passing said hot gas through the cementitious         material comprising ettringite, giving rise to an endothermic         dehydration and constituting a heat storage phase.

Passing the hot gas into the cementitious material makes it possible to bring said material to a temperature between 30° C. and 100° C. It is assumed that the cementitious material comprising ettringite is at atmospheric pressure. The decomposition temperature of ettringite increases greatly with the water vapour pressure. For example, if the material was subjected to a water vapour pressure of 8000 Pa, the storage temperature of said material could reach 80° C., without decomposing the ettringite. The heat storage phase mentioned during the step of passing the gas through the cementitious material, corresponds to the phase of charging heat into the cementitious material. The term “directly” means that the hot gas passes through the cementitious material comprising ettringite without the aid of any specific transportation device, such as for example pipes that would be placed around and/or within said material. The gases pass through the entire volume of the cementitious material, owing to its good porosity.

Advantageously, the heat storage phase extends over a period of several days, said period being dictated by the amount of cementitious material used in a system according to the invention. More specifically it is the period of the phase of charging heat into the material.

Preferentially, a storage/withdrawal process according to the invention comprises, in the heat withdrawal phase, the following steps:

-   -   a step of wetting the heat transfer fluid by means of the         wetting device in order to obtain a wetted gas,     -   a step of directly passing said wetted gas through the         cementitious material comprising ettringite and which adsorbs         water vapour then is hydrated,     -   a step of exothermic reaction which generates heat transported         by the heat transfer fluid, converting the initially cold and         wet gas into a hot and dry gas.

It is the restoring of this dry and hot gas that constitutes the production of heat.

Preferentially, a storage/withdrawal process according to the invention comprises a step of removing the carbon dioxide in the heat transfer fluid before it passes through the cementitious material comprising ettringite in order to prevent a carbonation reaction of said material. Indeed, if the heat transfer fluid is not an inert gas, it may be capable of transporting CO₂. In order to prevent a carbonation of the cementitious material comprising ettringite, which would be greatly detrimental to the efficiency of a system according to the invention, it is necessary to trap the CO₂ before it comes into contact with said cementitious material. A membrane may thus advantageously be used to stop the CO₂ molecules upstream of this cementitious material. The term “directly” means that the wetted gas passes through the cementitious material comprising ettringite without the aid of any specific transportation device, such as for example pipes that would be placed around and/or within said material. The gases pass through the entire volume of the cementitious material, owing to its good porosity.

Another subject of the invention is a cementitious material comprising ettringite for creating a storage/withdrawal system according to the invention.

Advantageously, a cementitious material according to the invention comprises an ettringitic binder, the porosity and the permeability of which are increased by chemical foaming and/or by mechanical foaming. Chemical foaming consists of additions of aluminium powder or of other foaming agents, and mechanical foaming consists of additions of surfactant and of a mechanical mixing. A high permeability combined with a high porosity of the ettringitic material favours the accessibility of the water vapour to the ettringitic crystals, and therefore the effectiveness of the process for storing heat in the material.

An energy storage/withdrawal system according to the invention has the following advantages:

-   -   A storage density considerably greater than that of the existing         low-temperature storage/withdrawal systems. The ettringitic         material has the advantage of combining both energy storage by         physisorption and storage by chemisorption. This gives it a high         storage density relative to the low-temperature sensible,         latent, physical sorption or chemical sorption heat storage         material.     -   The use of ettringite does not require thermal insulation during         the intermediate storage phase. In a storage/withdrawal process         according to the invention, there is an active heat charging         phase, a heat conservation phase and a heat withdrawal phase,         said active phase and said conservation phase constituting the         heat storage phase. The conservation phase may generally last         for a long time, around several months for a seasonal storage.         With ettringite, the heat stored by chemical reaction is         conserved while the material is insulated from water in liquid         or vapour form.     -   Such a system is both suitable for the short term and the long         term. The lack of need for thermal insulation makes it efficient         for long-term storage phases unlike the existing         storage/withdrawal systems.     -   Such a system enables good control of the heat storage phase.         The heat charging phase (desorption and dehydration) may be         carried out in several steps staggered over time, making the         storage/withdrawal system suitable for the seasons where the         sunshine is fleeting. Several days are needed to obtain a         complete dehydration of the material depending on the size of         the material used, and therefore to completely charge said         material with heat. This dehydration may be carried out         intermittently, the sunshine may for example partially dehydrate         the material on the first day, and may complete the dehydration         over the following days with a possibility of discharging heat         if need be.     -   Such a system enables a good control of the heat withdrawal         phase. As during the heat charging phase, the heat stored may be         returned in several steps, by controlling the amount of gas         adsorbed.     -   The ettringite has a low storage temperature, of the order of         60° C. This temperature may easily be achieved by means of a         conventional solar collector.     -   The ettringitic material is a monolithic material, which is         perfectly suitable for use in a facility in the form of walls         and/or bricks and/or partitions, unlike a powder bed. The         cementitious material may either be self-supporting or be used         as a load-bearing structure within an edifice consisting of         several materials. Indeed, its compressive mechanical strength         is between 0.1 MPa and 70 MPa.     -   It may lead to long heat withdrawal phases. Indeed, the         physicochemical combination of the storage process gives the         material a long discharging phase. During the initial         heat-discharging phase it is the physical adsorption (exothermic         reaction) which leads to the increase in the temperature over         several hours, then the temperature is maintained or even         increased by the exothermic hydration, of which the kinetics are         slow and may last several days.

Given below is a detailed description of a preferred embodiment of an energy storage/withdrawal system according to the invention and of the associated storage/withdrawal process, by referring to the following figures:

FIG. 1 is a general schematic view of a storage/withdrawal system according to the invention,

FIG. 2 is a schematic view of the part of the system from FIG. 1 required for the heat storage phase,

FIG. 3 is a schematic view of the part of the system from FIG. 1 required for the heat withdrawal phase,

FIG. 4 is a schematic view of a thermochemical reactor of a storage/withdrawal system according to the invention,

FIG. 5 is a diagram illustrating an example of the variation of the temperature in a cementitious material of a storage/withdrawal system according to the invention, during a heat withdrawal phase,

FIG. 6 is a diagram illustrating an example of the variation of the temperature in a cementitious material of a storage/withdrawal system according to the invention, during two heat withdrawal cycles.

In order to facilitate the reading of the detailed description, a “storage/withdrawal system” will be denoted under the simple designation “system”. Similarly, a “storage/withdrawal process” will be denoted under the simple designation “process”.

A cementitious material 1 of a system according to the invention is intended to form the constituent material of the walls and/or various partitions of an industrial facility or of a domestic dwelling. In order to describe the operating principle of such a system and the various steps of the associated process, the detailed description will focus on a system comprising a thermochemical reactor, the structure and the physicochemical properties of which are representative of those of the walls or partitions that would be formed by this cementitious material.

By referring to FIG. 1, a system 100 according to the invention comprises a circuit 8 for circulation of a heat transfer fluid, comprising a source 2 of said fluid, a wetting device 3, a heating device 4, the thermochemical reactor 5 comprising the cementitious material 1, and an outlet 6 of this heat transfer fluid. In the example considered, the source 2 of heat transfer fluid is a nitrogen cylinder. The wetting device 3 comprises at least one bubbler 7 suitable for charging the nitrogen with water, it being possible for said water to be present in the form of a liquid or vapour. The circuit 8 has at least one flowmeter 9 that makes it possible to measure the nitrogen flow rate and therefore to control it. The heating device 4 comprises at least one solar collector or another source of preferentially renewable energy able to recover heat over a temperature range between 30° C. and 100° C. The heating device 4 and the wetting device 3 may be activated independently of one another and may therefore operate alternately. The circuit 8 passes firstly through the wetting device 3, then through the heating device before passing through the thermochemical reactor 5, the fluid resulting from the passage through said reactor 5 then being discharged to the atmosphere in order to heat a room or a premises.

By referring to FIG. 4, the thermochemical reactor 5 comprises a shell 10 consisting of a cylindrical wall enclosing a monolithic cementitious material 1 comprising a large mass fraction of ettringite, of between 20% and 90%. This material 1 constitutes a cylindrical block having the following characteristics:

-   -   a high porosity of between 10% and 90%,     -   a permeability of between 10⁻¹⁷ m² and 10⁻¹¹ m²,     -   a mechanical strength of between 0.1 MPa and 70 MPa.

The shell 10 is split into an inner shell 11 formed by a layer of PVC (polyvinyl chloride) that comes into contact with the outer lateral surface of the cementitious block 1 and an outer shell 12 formed by a layer of glass wool surrounding the PVC layer 11 and being in contact therewith. The PVC layer 11 acts as a water insulation material and the glass wool layer 12 acts as a thermal insulation material. This thermochemical reactor 5 has an inlet 13 for nitrogen originating from the heating device 4 or from the wetting device 3, and a nitrogen outlet 14 after this nitrogen has passed through the monolithic cementitious block 1. Thermal sensors, such as for example thermocouples, and water sensors may be inserted into the cementitious block 1 in order to enable the control of the temperature of said block 1 and of the water vapour pressure in the reactor 5, over time.

All the measurement elements present in the system 100 make it possible to acquire the operating parameters of said system 100, which may then be recorded and processed by a computer 15.

A system according to the invention makes it possible to become charged with heat, then to store this heat over an unlimited given period of time, before restoring it to order, when the heating requirements take effect.

A storage/withdrawal process using a system 100 according to the invention comprises the following steps:

-   -   A—In the charging phase, illustrated in FIG. 2 (this phase may         for example take place in summer where the amount of sunshine is         considerable)         -   A step of heating the heat transfer fluid 2 in order to             obtain a hot gas, the temperature of which is between 50° C.             and 70° C., and preferentially is equal to 60° C. This             heating step is carried out by means of the heating device             4, and in particular by means of the solar collector.         -   A step of passing said hot gas through the cementitious             material 1 comprising ettringite, giving rise to an             endothermic dehydration and constituting a heat storage             phase. Indeed, the nitrogen is thus heated between 30° C.             and 100° C. before passing through the cementitious block 1             housed in the thermochemical reactor 5, the passage through             said block 1 being facilitated by the high porosity thereof.             The nitrogen then heats the cementitious block 1 of             ettringite, which is dehydrated, simulating the heat storage             phase. The endothermic desorption of water over the             ettringite, which is a physicochemical reaction, makes it             possible to store heat. The thermal energy is thus stored             and conserved in the cementitious material 1, while said             material 1 remains insulated from water. For this charging             phase, the device 3 for wetting the nitrogen is not needed             and is not therefore activated. A complete charging phase             lasts around several days depending on the amount of             cementitious material 1 used and on the fluid flow rate. By             way of example, on average 3 days are required for several             kilograms of material and a nitrogen flow rate of 2 l/min.             The chemical part of the heat charging process (endothermic             dehydration) is linked to the reversible conversion of             ettringite to metaettringite, by loss of 18 water molecules             per ettringite molecule:

3CaO.Al₂O₃.3CaSO₄.3H₂O→3CaO.Al₂O₃.3CaSO₄.12H₂O+18H₂O

-   -   B— In the discharging phase, illustrated in FIG. 3 (this phase         may for example take place in winter where the heating         requirements are high)         -   A step of wetting the heat transfer fluid 2 by means of the             wetting device 3 in order to obtain a wetted gas. This             wetting step is carried out by means of the wetting device             3, and in particular, by means of bubblers. The gas             resulting from the wetting device 3 is thus charged with             water vapour. The wetted gas can pass through the monolithic             cementitious material 1 comprising ettringite since said             material 1 is porous and permeable.         -   A step of passing said wetted gas through the cementitious             material 1 comprising ettringite and which adsorbs water             vapour then is hydrated. The adsorption is a physical             phenomenon and the hydration is a chemical phenomenon. The             chemical part of the heat restoring process is linked to the             rehydration of metaettringite to ettringite by a gain of 18             water molecules per ettringite molecule:

3CaO. Al₂O₃.3CaSO₄.12H₂O+18H₂O→3CaO. Al₂O₃.3CaSO₄.30H₂O

-   -   -   A step of exothermic reaction at the cementitious material,             which generates heat transported by the heat transfer fluid             2, transforming the initially cold and wet gas into a hot             and dry gas. During this heat withdrawal phase, the heating             device 4 is not needed and is not therefore activated.

By referring to FIG. 5, the maximum temperature increase in the cementitious material 1 may for example reach 16° C., during a heat withdrawal phase. This temperature increase depends on the amount of material, on the relative humidity and on the flow rate of fluid used. Such a diagram shows that the heat withdrawal phase may last up to 3 days, which remains a long period relative to that of the systems using zeolite type materials, for which this period does not exceed 24 h.

FIG. 6 illustrates the reversibility of the storage process of a system according to the invention. The reversibility of the heat storage process is linked to that of the chemical reaction for dehydrating and rehydrating ettringite. 

1. System for storing/withdrawing thermal energy, comprising a monolithic cementitious material comprising a mass fraction of ettringite of greater than 20%, said material being surrounded by a thermal insulation material and a water insulation material, a source of a heat transfer fluid, a device for wetting said fluid in order to carry out a withdrawal phase of the system, a device for heating said fluid in order to carry out a storage phase of said system, an outlet of the heat transfer fluid from said material, wherein said system comprises at least one heat transfer fluid circulation circuit that passes through the heating device and through the wetting device, said circuit being supplied by the source of said fluid and leading to the cementitious material.
 2. System according to claim 1, wherein the heat transfer fluid is formed by an inert gas.
 3. System according to claim 1, wherein the source of the heat transfer fluid is placed upstream of the wetting device, which is itself placed upstream of the heating device, and in that said heating device is placed upstream of the cementitious material.
 4. System according to claim 1, wherein the wetting device and the heating device each operate to order, and can be activated independently of one another depending on the use of said system, either in storage phase, or in withdrawal phase.
 5. System according to claim 1, wherein the mass fraction of ettringite in the cementitious material is between 20% and 90%.
 6. System according to claim 1, wherein the cementitious material has a porosity of between 10% and 90%.
 7. System according to claim 1, wherein the cementitious material (1) has a permeability of between 10⁻¹⁷ m² and 10⁻¹¹ m².
 8. System according to claim 1, wherein the cementitious material (1) has a compressive mechanical strength of between 0.1 MPa and 70 MPa.
 9. System according to claim 1, wherein the water insulation material is formed by an impermeable wall intended to prevent water vapour from coming into contact with the cementitious material.
 10. System according to claim 1, wherein the thermal insulation material is formed by at least one material to be chosen from glass wool and polystyrene and in that said at least one impermeable material is placed around the water insulation material.
 11. System according to claim 1, wherein the wetting device comprises at least one humidifier suitable for charging said fluid with water.
 12. Process for storing/withdrawing thermal energy using a storage/withdrawal system in accordance with claim 1, wherein, in the heat storage phase, it comprises the following steps: a step of heating the heat transfer fluid by means of the heating device in order to obtain a hot gas, the temperature of which is above 30° C., a step of directly passing said hot gas through the cementitious material comprising ettringite, giving rise to an endothermic dehydration of said material and constituting a heat storage phase.
 13. Process according to claim 12, wherein the heat storage phase extends over a period of several days, said period being dictated by the amount of cementitious material and the flow rate of fluid used in said process.
 14. Process according to claim 12, wherein, in the heat withdrawal phase, it comprises the following steps: a step of wetting the heat transfer fluid by means of the wetting device in order to obtain a wetted gas, a step of directly passing said wetted gas through the cementitious material comprising ettringite and which adsorbs water vapour then is hydrated, a step of exothermic reaction which generates heat transported by the heat transfer fluid, converting the initially cold and wet gas into a hot and dry gas.
 15. Process according to claim 12, wherein said process comprises a step of removing the carbon dioxide in the heat transfer fluid before it passes through the cementitious material comprising ettringite in order to prevent a carbonation reaction of said material.
 16. Material comprising ettringite for creating a system in accordance with claim
 1. 17. Material according to claim 16, wherein said material comprises an ettringitic binder, the porosity and the permeability of which are increased by chemical foaming and/or by mechanical foaming. 